Mass spectrometry is a technique used in the field of chemical analysis to detect and identify analytes of interest. The sample must first be ionised so that components may then be acted upon by electric fields, magnetic fields, or combinations thereof, and subsequently detected by an ion detector. Mass analysers are operated at low pressure to ensure that the trajectories of the ions are dominated by the applied fields rather than by collisions with neutral gas molecules. However, it is often convenient to use an ion source operating at atmospheric pressure. Consequently, neutral gas molecules and entrained ions from the source must be drawn into the vacuum system through a small aperture. Atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) are two common examples of such sources that are in widespread use.
The size and weight of a conventional atmospheric pressure ionisation (API) mass spectrometer is dominated by the pumping system, which is designed to maximise the amount of gas and entrained ions that can be drawn through the inlet, and at the same time maintain the pressure in the region of the mass analyser at a level consistent with its proper operation. A conventional bench-top instrument typically weighs approximately 100 kg and is coupled via a bulky vacuum hose to a floor-standing rotary pump, weighing an additional 30 kg. The power consumption can be more than a kilowatt, and relatively high levels of heat and noise are generated.
In vacuum systems, the flow of gas, Q, is given byQ=SP   (Eq. 1)where S is the speed of the pump, and P is the pressure. There are many different types of vacuum pumps, but in all cases the pumping speed is related to the size and weight of the pump. According to Eq. 1, a large pump is required to simultaneously achieve low pressure and high gas throughput.
Prior to the widespread adoption of turbomolecular pumps, oil diffusion pumps and cryo pumps were used to achieve the high vacuum conditions required for mass spectrometry. Oil diffusion pumps are mechanically simple, dissipate a lot of heat, must be mounted in the upright position, are usually water cooled, and operate with a foreline (backing) pressure of less than 1 Torr. Cryopumps offer very large pumping speeds but require a supply of liquid nitrogen, or a bulky helium compressor. Turbomolecular pumps are mechanically complex and consequently relatively expensive. However, they are compact, generally air cooled, and can be mounted in any orientation. In addition, small and medium sized turbomolecular pumps tolerate a high foreline pressure.
Pumps capable of achieving low and medium vacuum pressures are needed for initial evacuation, direct pumping of vacuum interfaces, and providing foreline pumping. Such pumps are often referred to as roughing pumps, or backing pumps when used for foreline pumping. Oil-filled rotary vane pumps are universally used in conventional instruments. These are typically heavy, bulky, noisy, and require frequent servicing. Consequently, they are not housed within the main body of the instrument. Very lightweight and compact diaphragm pumps are available for low gas load applications. Although often used to provide foreline pumping for small turbomolecular pumps, they are not suitable for direct pumping of vacuum interfaces operating at or near 1 Torr.
Early system architectures, designated as Type A and Type B, are shown in FIGS. 1 and 2, respectively. In FIG. 1, gas and entrained ions from the atmospheric pressure ion source pass directly into the analysis chamber via an inlet orifice. A large high vacuum (HV) pump is required to pump the full gas load at the low pressure required for proper operation of the mass analyzer. The pressure in the foreline must be maintained at an intermediate pressure by a roughing pump, as high vacuum pumps cannot exhaust directly to atmospheric pressure. Even with a relatively large high vacuum pump, the orifice needs to be very small to limit the gas flow to a manageable level. For example, a 25 μm diameter inlet orifice requires a high vacuum pump with a speed of approximately 1000 L/s.
In FIG. 2, differential pumping is used to partly separate the tasks of pumping large volumes of gas and achieving the low pressures required by the mass analyser. A larger inlet orifice can be tolerated as the majority of the gas load is pumped at a relatively high pressure by the first chamber pump. Electrostatic lenses are used to focus the ions towards the inter-chamber aperture, thereby substantially increasing the concentration of ions in the gas flowing into the second chamber. However, the distance between the inlet orifice and the inter-chamber aperture must be kept short as ions are scattered when they collide with neutral gas molecules. The two high vacuum pumps are collectively less massive than the single pump that would be required if a Type A architecture had been adopted.
It was later appreciated that molecular beam techniques and principles could be applied in the design of API mass spectrometers [M. Yamashita and J. B. Fenn, J. Phys. Chem. 88, 1984, 4451-4459]. The general arrangement is shown in FIG. 3, and denoted as Type C. A portion of the gas flow through the inlet orifice is transferred to the second chamber through a skimmer that samples from the centre of the initial free jet expansion. The fraction of the total gas load transmitted by the skimmer is determined by the pressure in the first chamber, the area of the skimmer inlet, and its position relative to the inlet orifice. In the original embodiment of this idea, the first chamber was pumped to 10−3 Torr using a 1000 L/s diffusion pump, the inlet orifice was 70 μm in diameter, the skimmer was 4 mm in diameter, and the second chamber was pumped by two pumps with a total speed of 3000 L/s. Although a small fraction of the ions are transferred to the second chamber, the beam is well-collimated, which is ideal for efficient coupling to the mass analyser. Disadvantages of this arrangement are that large clusters and droplets can be transmitted through the skimmer, and free analyte ions condense with solvent or ambient water vapour during the adiabatic expansion.
An API mass spectrometer employing a differentially pumped radio frequency (rf) ion guide was described in 1987 [J. A. Olivares, N. T. Nguyen, C. R. Yonker, and R. D. Smith, Anal. Chem., 59, 1987, 1230-1232]. A schematic representation of this instrument is shown in FIG. 4, and designated as Type D. During the past twenty years, designs based on this architecture have been universally adopted by manufacturers of conventional instruments.
The trajectories of the ions are confined by an rf quadrupole ion guide as they transit the second chamber. In the case of a quadrupole ion guide, the field is generated by four rods arranged symmetrically about a common axis. The voltage applied to each rod is required to oscillate at rf, with the waveforms applied to adjacent rods having opposite phase.
The first chamber is operated at approximately 1 Torr, which is conveniently achieved with a rotary pump. As shown in FIG. 4, the same rotary pump can be used to pump the first chamber and provide foreline pumping for the two high vacuum pumps. At this pressure, the skimmer profile is much more critical, as the high gas density can result in a shock structure that disrupts the continuum flow of the jet. Typically, a skimmer with a 0.75-2.5 mm diameter inlet is located several millimeters downstream of a 200-350 μm diameter inlet orifice. An electrostatic lens can be placed around the skimmer to focus ions into its entrance, and thereby increase the ion-to-neutral gas ratio. In some systems, nearly all the ions are transmitted to the next stage. However, in view of the problems described in connection with FIG. 3, namely the formation of cluster ions and the transmission of droplets, some manufacturers have preferred to place a sampling orifice or cone downstream of the Mach disc, often in such a way that there is no line-of-sight trajectory from the inlet aperture. When this is the case, an electric field is applied so as to attract ions towards the sampling orifice or cone.
An alternative embodiment of the Type B architecture that incorporates an rf ion guide rather than electrostatic ion lenses is shown in FIG. 5, and designated as Type E. The first chamber contains an ion guide and is pumped to a pressure of 10−4-10−2 Torr by a suitable high vacuum pump. Gas and ions pass through the inlet orifice whereafter the ion trajectories are constrained such that they may pass through the aperture between the first and second vacuum chambers.
Recently, one manufacturer has abandoned the traditional orifice-skimmer interface in favour of a short ion guide operating at high pressure. The arrangement is shown in FIG. 6 and designated as Type F. The size of the inlet orifice, the field radius of the first ion guide, and the pressure in the first chamber are chosen such that the free jet expansion is largely contained within the ion guide. A substantial fraction of the ion flux is captured and transmitted to the second vacuum chamber whereas the neutral gas escapes through the gaps between the rods. The first ion guide operates at a pressure of several torr, and as a result is followed by a second ion guide in the next vacuum chamber, which removes more of the gas load before the ions are mass analysed.
Increasingly, small, light-weight analytical instruments are required for industrial process monitoring, security applications, the detection of toxic or illicit substances, and deployment in remote or hazardous environments. In addition, the growing amount of equipment being used by analytical chemists in traditional laboratories has forced greater consideration of factors such as the linear bench space occupied by instruments, heat and noise generation, initial purchase price, and operational costs. Consequently, there is a need for a miniature API mass spectrometer that, although much smaller than a conventional system, is capable of a useful level of sensitivity. While the detection efficiency of a particular instrument depends on the details of its design, the ultimate sensitivity is limited by the amount of gas and entrained ions that can be drawn through the inlet. Unfortunately, even a modest scaling down of the system architecture used for conventional instruments results in a significant reduction in the gas load that can be tolerated. Accommodating all the pumps within a single, small enclosure is a particular difficulty, as the size and weight of the pumps commonly used to achieve intermediate vacuum do not scale favourably.